FIG. 1 illustrates in cross-section a typical wind turbine rotor blade 10. The rotor blade 10 comprises an outer shell 12 that is fabricated from first and second half shells 14, 16. The half shells 14, 16 are laminated structures that are moulded from glass-fibre reinforced plastic (GRP). Each half shell 14, 16 comprises inner and outer skins 20, 18 with structural reinforcing elements such as longitudinally-extending spar caps 22 (also referred to as beams, bearing structures, girders etc) formed from pultruded strips of carbon fibre reinforced plastic (CFRP) arranged between the inner and outer skins 20, 18. Foam panels 24 forming sandwich panel cores typically fill the gaps between the structural elements.
The half shells 14, 16 are moulded in separate mould halves 25, an example of which is illustrated in plan view in FIG. 2. Once each half shell 14, 16 has been moulded, the two half shells 14, 16 are brought together by closing the mould, and the half shells 14, 16 are bonded together to form the complete blade 10.
To form a half shell 14, 16, one or more outer dry glass-fibre cloth layers are placed on a mould surface of the mould half 25. These layers will later form the outer skin 18 of the blade. Structural elements, including the spar caps 22 and the foam panels 24, are then laid up on the cloths. One or more inner dry glass-fibre cloth layers are then placed over the structural elements, and these will later form the inner skin 20 of the blade.
Next, the elements of the half shell 14, 16 are covered with an airtight bag to form an evacuation chamber encapsulating all of the elements. The chamber is evacuated and a supply of liquid resin is connected to the chamber. The resin is introduced into the mould half 25 and infused through and between the elements.
Once the resin has been infused, the assembly undergoes a curing cycle to harden the resin, during which time the mould assembly 25 is heated. The assembly 25 may be heated by external heating elements, or alternatively heating elements may be embedded in the mould to provide a heatable mould.
In such heatable moulds, it is known to provide a heat distribution layer. For example, U.S. Pat. No. 3,387,333, describes a metallic layer arranged between the heating elements and the mould surface which distributes heat from the heating elements uniformly across the mould surface to ensure even heating of the half shell.
However, the half shell 14, 16 comprises different elements which, being made of different materials and/or having different thicknesses, have different heat capacities. In particular, carbon-containing elements, such as the spar caps 22, have a relatively high heat capacity, while other elements, such as the foam panels 24 between the carbon elements, have a relatively low heat capacity.
To cure the resin fully, the mould 25 must be heated until all the elements of the half shell 14, 16 have reached a required curing temperature Tcure. This curing temperature is the temperature that is required to cure the resin, and is typically between approximately 60° C. and approximately 120° C. It will be appreciated that the spar caps 22, having high heat capacity, require more energy to reach the curing temperature than the foam panels 24, which have a low heat capacity. Excess heat energy is therefore supplied to the foam panels 24 causing them to overheat as the spar caps 22 are brought up to the curing temperature. The curing process is therefore energy inefficient. Furthermore, in extreme cases, the foam panels 24 may exceed a maximum safe exposure temperature Tmax, (between approximately 120° C. and approximately 150° C.), which may damage the foam panels 24, and compromise the structural integrity of the blade 10.
This problem can be mitigated to some extent by providing a plurality of heating control zones 26 within the mould 25, as illustrated in FIG. 2. Each heating control zone 26 comprises a heating element that can be independently controlled, so that more heat energy is supplied to some heating control zones 26 than others. Areas of higher heat capacity can therefore be supplied with more heat than those of lower heat capacity. However, the control systems required for independent control of each heating control zone 26 are costly, and the greater the number of heating control zones 26, the higher the cost of the mould 25. The resolution of heating control zones 26 is therefore limited by cost, and typically a control zone 26 must be several square meters in area. In practice, a single control zone 26 must therefore support several elements of varying heat capacity. Thus it is not practical to use heating control zones 26 to eliminate altogether the problem of excess heat being supplied to elements of low heat capacity, such as the foam panels 24.
Against this background, it is an object of the present invention to provide a cost-efficient mould for a wind turbine component that addresses or mitigates the above problem.